*{
	margin: 0px;
	padding: 0px;
}
/*
 * left与right不能同时使用
 * body默认margin 8px
 */
/*div{
	position: absolute;
	left: 100px;
	top: 100px;
	width:100px;
	height: 100px;
	background-color: red;
}*/
/*========================line1=======================*/
/*.demo{
	position: absolute;
	left: 100px;
	top: 100px;
	width: 100px;
	height: 100px;
	background-color: red;
	opacity: 0.5;
}
.box{
	width: 150px;
	height: 150px;
	background-color: green;
}*/
/*=================line2===================*/
/*.demo{
	position: relative;
	left: 100px;
	top: 100px;
	width: 100px;
	height: 100px;
	background-color: red;
	opacity: 0.5;
}
.box{
	width: 150px;
	height: 150px;
	background-color: green;
}*/
/*=====================line3======================*/
/*.wrapper{
	margin-left: 100px;
	width: 200px;
	height: 200px;
	background-color: orange;
}
.content{
	position: relative;
	margin-left: 100px;
	width: 100px;
	height: 100px;
	background-color: black;
}
.box{
	position: absolute;
	left: 50px;
	width: 50px;
	height: 50px;
	background-color: yellow;
}*/
/*=====================line4======================*/
/*.wrapper{
	margin-left: 100px;
	width: 200px;
	height: 200px;
	background-color: orange;
}
.content{
	margin-left: 100px;
	width: 100px;
	height: 100px;
	background-color: black;
}
.box{
	position: relative;
	left: 50px;
	width: 50px;
	height: 50px;
	background-color: yellow;
}*/
/*===================================line5====================*/
/*div{
	position: fixed;
	left: 0;
	top: 300px;
	width: 50px;
	height: 200px;
	background-color: red;
	color: #fffff;
}*/
/*================================line6=======================*/
div{
	position: fixed;
	left: 50%;
	top: 50%;
	width: 100px;
	height: 100px;
	background-color: red;
	margin-left: -50px;
	margin-top: -50px;
}
